According to the new Tier List (Super Street Fighter 4 Tiers ? Character Rankings : EventHubs.com) these are the bad (< 5) matchups for Dhalsim:
Cammy: Beaten by chun, honda, bison, guile, balrog, ryu, ken, zangief, dee jay
Abel: Beaten by chun, bison, cammy, fei long, adon, hawk
Rufus: Beaten by bison, guile, balrog, cammy, sagat, seth, ken, zangief, dee jay
Adon: Beaten by chun, honda, bison, balrog, cammy, sagat, hawk
Vega: Beaten by chun, honda, bison, guile, balrog, cammy, ryu, abel, rufus, akuma, adon, ibuki
Viper: Beaten by honda, bison, balrog ,cammy ,ryu, rufus, blanka, zangief, ibuki
Fuerte: Beaten by honda, bison, balrog, cammy, ryu, rufus, seth, akuma ,fei long
Gouken: Beaten by chun, bison, guile, balrog, cammy, ryu, abel, rufus, fei long, blanka, ibuki, vega, cody, gen
According to this list, the best secondary characters a Dhalsim player could play are, in my opinion:
1) Bison: Beats 8 (all) of Dhalsim’s bad matchups
2) Cammy: Beats 7 of Dhalsim’s bad matchups, aside from herself
3) Balrog: Beats 7 of Dhalsim’s bad matchups, aside from Abel